Since 1998, the Environmental Protection Agency has been trying to enforce the Clean Air Act’s “good-neighbor” provision, which obliges states not to foul each other’s air. Today (Tuesday), the Supreme Court will hear arguments over whether the agency’s latest attempt is legal or whether it is an overly grasping application of authorities Congress never gave it.

At stake are long-delayed regulations on ozone and particulate pollution that increase the incidence of heart attacks, asthma and other illnesses — and large amounts of money that may have to be spent to abate those airborne hazards.

The Clean Air Act empowers the EPA to restrict pollution that will “contribute significantly” to a major air-quality problem somewhere else. Aside from some technical issues, the case, EPA v. EME Homer City Generation L.P., turns on the definition of “significantly.”

The EPA found that one state’s contribution to an air-quality problem is significant insofar as it is solvable. The agency mandated reductions based not just on how much pollution each state emits but also on its capacity to cut back.

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This policy is reasonable. The federal government should be refereeing national air pollution problems, particularly those that states can’t be expected to solve on their own. And if the feds are going to intervene, their rules should mandate the least disruptive, most efficient path to reducing pollution.

But there’s a problem, say the EPA’s adversaries: The law doesn’t give the agency the flexibility to consider costs in the way it did. Rather, it must base regulations exclusively on how much pollution each state contributes. This interpretation may seem fairer to people in one state who don’t want to pay to reduce pollution when they’re producing less of it than are people in other states. But it could lead to some very undesirable results. The EPA may, for example, have to require certain states to spend huge amounts of money to reduce pollution by small or negligible amounts.

There’s no doubt the EPA’s approach is the better policy, and the law’s wording is imprecise enough that the agency should get a fair amount of latitude in its application of this part of the Clean Air Act. But this is not the only case the Supreme Court will hear soon involving awkward wording in the act. The most rational thing would be for Congress to update the law — not to gut its programs but instead to prescribe neater solutions to the country’s serious pollution problems. However, it has been years since lawmakers managed to pass any major environmental legislation.